The present invention is directed to a dental turbine drive which contains a rotor disk mounted for rotation by bearings in a housing of a handpiece. The rotor disk has a plurality of turbine paddles or blades which are charged with compressed air via a nozzle disposed in the housing, and the turbine drive includes control means for governing the speed of the rotor, which control means is disposed in the flow path of the exhaust air.
U.S. Pat. No. 3,865,505 discloses a turbine drive with control means, wherein a valve that is controlled dependent on the volume throughput of the returned air is arranged in the delivery channel of the compressed drive air. When the speed of the turbine drops due to an external moment, such as a load being applied to the turbine, the valve in the delivery channel is then opened and a greater air volume is conducted to the rotor disk of the turbine as a result of this operation. The regulation of the admission air or the delivery channel is dependent on the return air and can occur in various ways in the known turbine, including a spring-loaded slide in the return air channel that controls the valve in the admission air channel or by a diaphragm/aneroid diaphragm arranged in a return air channel that adjusts a piston that controls the flow in the admission air channel or line. The speed can be kept constant independent of the load with this known control means. Among other things, the apparatus is affected by the disadvantage that relatively great forces that can be exerted in the exhaust air channel only upon utilization of the volume and mass are required for throttling the overall airstream. Therefore, an unstable control behavior of the turbine is created. In addition, the known apparatus requires a relatively large structure inside the turbine handpiece and problems with integration of the control into the handle may occur.
U.S. Ser. No. 08/261,149, filed Jun. 14, 1994, whose disclosure is incorporated herein by reference thereto and which claims priority from the same German Application resulting in German Patent 43 20 532, discloses a dental turbine drive, wherein an adjustment or control means is arranged on the rotating part of the rotor disk for governing the speed. This adjustment means changes its shape and/or position upon rotation of the rotor disk as a consequence of the centrifugal forces that are applied thereto and will change the effective cross section of a discharge channel with increasing speeds causing a decrease in the effective cross section and decreasing speeds causing an increase in the effective cross section.
The control procedure is essentially based on the fact that the actuating power required for regulating results from the speed-dependent centrifugal forces that act on the inherently passive control element arranged on the rotor so that the effective flow cross section for the airstream is varied based on changes in the speed. Given a prescribed, rated speed, the maximum actuating powers and regulating distance that can be achieved can only be influenced via the mechanical properties of the control element, which include the mass, the modulus of elasticity and shape. Therefore, the control element can be influenced to only a limited extent. This is especially true given the employment of commercially standard elastically deformable rings, such as O-rings. The deviation from the desired rated speed that occurs can be relatively great under certain circumstances.